While generally useful on their own inherent merits, the physical properties of styrene polymer/thermoplastic elastomer (i.e., SP/TE) polyblend systems are considerably improved when an optimum level of cross-linking is achieved in the elastomeric constituent (i.e., "rubber" or rubbery portion) of the TE. This is particularly advantageous and practical when the SP constitutent of the SP/TE polyblend is general purpose (homo)polystyrene (i.e., "GP-PS").
The SP's employed are generally polymers and copolymers of alkenyl aromatic monomers of the Formula: EQU CH.sub.2 .dbd.CGAr, (I)
wherein G is selected from the group consisting of hydrogen and methyl and Ar is an aromatic radical, including various alkyl and halo-ring-substituted aromatic units of from 6 to 10 carbon atoms. Styrene (i.e., "St") is ordinarily the most advantageous and oftentimes preferred species of the Formula (I) monomers to utilize. Others that are frequently quite satisfactory include: .alpha.-methyl styrene (i.e., "MeSt"); vinyl toluene (i.e., "VT"); vinyl naphthalene (i.e., "VNPth"); the dimethyl styrenes (i.e., "diMeSt's"), t-butyl styrene (i.e., "t-BuSt"); the several chlorostyrenes (such as the mono- and dichloro-variants--i.e., "ClSt" and "diClSt"); the several bromostyrenes (such as the mono- and dibromo-varients--i.e., "BrSt" and "diBrSt"); and so forth.
Copolymeric SP's can be copolymerizates of one or more Formula (I) monomers, particularly St, with one (or even mixtures) of other addition-polymerizable monoethylenically unsaturated comonomers that are copolymerizable with St including, by way of illustration and not limitation; acrylonitrile (i.e., "VCN") and methacrylonitrile; vinyl chloride (i.e., "VCl") and other vinyl halides; vinylidene chloride (i.e., "VeCl"); acrylic acid (i.e., "HAcr") and its addition-polymerizable esters; methacrylic acid (i.e., "HMeAcr") and its addition-polymerizable esters; various vinyl organic esters such as vinyl acetate, vinyl propionate, etc.; and so forth.
The SP's utilized may also be the rubber-modified interpolymerized products of graftable pre-formed elastomers and monomers of the Formula (I). Typical of these are the so-called high impact polystyrenes (i.e., "HIPS"). When use is made for the SP's of rubber-modified, impact grade plastics products, it is customary for them to be prepared by incorporation in the composition of from, say, 1-20 wt. percent, of an unsaturated, graft-copolymerizable stock of natural or synthetic rubbery elastomers (as hereinafter more fully described) for interpolymerization with the monoethylenically-unsaturated monomer in the reaction mass; all according to established procedures. The modifying rubber in current vogue is polybutadiene or a polybutadiene derivative; although, if desired, natural rubbers may be employed as may styrene/butadiene polymers (as, for example, of the well-known "GRS"-type), polyether elastomers, etc.
It is of general good advantage when copolymeric SP's are employed for at least about 60 percent by weight (i.e., "wt. %"), based on copolymer weight, of Formula (I) monomer(s) that are copolymerizable with St to be copolymerized in the polymer molecule. More advantageously, this is at least about 80 wt. percent, with the balance of copolymerized ingredients being desired comonomer(s) that are copolymerizable with St.
An almost invariable and desirable characteristic of TE's is their inherent combination of the natural flexibility and impact resistance of rubbers with the normally-usual strength and easy-processability of thermoplastics, coupled with features of frictional properties and hardness that are generally intermediate those of conventional rubbers and thermoplastics.
Generally, the TE's may be characterized as "rubbery (or elastomeric) "block" copolymers which, sometimes, are even in at least approximate if not actual "graft" copolymer form. They are, insofar as concerns the presently-contemplated SP/TE polyblends, various sorts and arrangements of a "rubbery" or "elastomeric" center or other possible "backbone" or "substrate" constituent (i.e., and "EL") to and upon which are attached the end or otherwise connected "blocks" of interpolymerized SP (i.e., "IPSP") units. In all cases, in order for an adequate inherent potential for cross-linkability to therein exist, the TE's that are utilized must contain and exhibit a greater or lesser extent or degree of unsaturation therein.
Most, if not literally all, of the presently known varieties of TE's are made by ionic, generally anionic, solution polymerization using an organometallic catalyst, such as sec.-butyl-lithium, n-butyl-lithium or the like or equivalent catalysts, as explained in Reference Number 12 (i.e., "Ref. 12") in the following "LISTING OF REFERENCES" Section of this Specification. Refs. 1 and 2 also deal with this.
Typical architecture(s) of TE's are represented by the Structures: ##STR1## and so on and so forth, all wherein "n" is an integer which, usually, is 1 but can alternatively depend in numerical value on the particular molecular weight (generally a weight average measurement--i.e., "Mw") or chain length of given interconnected EL units in the instances when they are ultimately so joined or formed.
Structure (S I) is quite common, being represented by that commercial variety available from "THE GENERAL TIRE AND RUBBER COMPANY" made from PS and polybutadiene (i.e., "PBu") in the block copolymer form IPSP-PBu-IPSP containing about 40 weight percent PS and having a Mw of about 550,000 (Ref. 6). Structures (S III) through (S V), inclusive, are at least by analogy more or less in the nature of graft copolymers. Structures (S IV) and (S V) are often times referred to as "star-blocks" or "radial-blocks". A good example of a Structure (S V) star-block is that obtainable under the trade-designation "SOLPRENE.RTM.", as described in Ref. 9. This is a radial block (IPSP).sub.4 -PBu of varying IPSP:PBu ratio and composition in differing Mw products. "KRATON G.RTM." is explained in Refs. 10 and 11 and typifies a commercially available Structure (I) material which is a IPPS (hydrogenated)-PBu-IPPS triblock of varying IPPS:PBu ratio, composition (including mineral oil--i.e. "MO"--content) and M.sub.w. Structure (S VI) diblock copolymers often have what is referred to as a "tapered" interpolymerized construction of varying M.sub.w and IPSP:PBu ratio.
The EL blocks, often referred to as being the "soft" ones in IPSP/EL interpolymers, provide the rubbery properties to the interpolymer. The IPSP blocks, often referred to as being the "hard" ones in the subject interpolymers, tend to associate or conglomerate into glassy domains. These effectively function as "cross-links", at least insofar as restricting the free movement of the macromolecular TE chains is concerned. The IPSP blocks also give the IPSP/EL product at least the bulk of its tensile strength. The IPSP block domains tend to disappear when softened by heat; re-forming when the interpolymer product is cooled. This, advantageously, allows processing and fabrication of the material according to the various techniques and procedures customarily followed for normal thermoplastics.
Frequently, the overall ratio of "hard" IPSP blocks to "soft" EL blocks in the TE structure is about 2:1 by respective chain(s) proportion reckoning. This is particularly so in strictly and somewhat classic types of "block" copolymers represented by the Structure (S I) and the more or less "graft" styles represented by Structures (S II) and (S III). It may also apply to many SP/TE interpolymers of the (S IV), (S V) and even (S VI) Structures. Useful IPSP/EL materials can be comprised of as little as about 20-25 weight percent or so of the EL constituent. Often, however, this EL content may be on the order of at least 45-50 weight percent and even greater.
The EL utilized for preparation of the TE's may be selected from a wide variety of generally sulfur-vulcanizable materials. It can, for example, be natural rubber (otherwise known as Hevea Brasiliensis). Much more often, however, it is a conjugated diolefine (homo)polymer synthetic rubber (or elastomeric inter-, or co-polymer composition of between about 25 and about 90 weight percent) of a 1,3-diene of the Formula: EQU H.sub.2 C:CR--CH:CH.sub.2, (II)
wherein R is selected from the group consisting of hydrogen, chlorine and methyl radicals.
Such conjugated diolefine polymer synthetic rubbers are polymers of: butadienes-1,3, e.g., butadiene-1,3; isoprene; 2,3-dimethylbutadiene-1,3; and copolymers of mixtures thereof; and copolymers of mixtures of one or more such butadienes-1,3, for example, of up to 75 weight percent of such mixtures of one or more mono-ethylenic compounds which contain a EQU CH.sub.2 .dbd.C.dbd. (IIA)
grouping, wherein at least one of the disconnected valences is attached to an electronegative group, that is, a group which substantially increases the electrical dissymmetry or polar character of the molecule.
Examples of compounds which contain the Formula (IIA) grouping and are copolymerizable with butadienes-1,3 are: the Formula (I) monomers, especially St; the unsaturated carboxylic acids and their esters, nitriles and amides, such as acrylic acid, methyl acrylate, ethyl acrylate, methyl methacrylate, acrylonitrile, methacrylonitrile, methacrylamide; vinylpyridines, such as 2-vinylpyridine, 2-methyl-5-vinylpyridine; methyl vinyl ketone, and methyl isopropenyl ketone--all of which besides those above mentioned in connection with the SP's are also compolymerizable with St.
Examples of such conjugated diolefine polymer synthetic rubbers of polybutadiene, polyisoprene, butadiene/styrene copolymers (i.e., "SBR") and butadiene/acrylonitrile copolymers. The synthetic rubber may be solution-prepared or emulsion-prepared, be it a stereo-specific variety or otherwise.
Other conventional unsaturated sulfur-vulcanizable rubbers may also be used as the EL constituent, such as "EPDM" (rubbery terpolymer of ethylene, propylene and a copolymerizable non-conjugated diene such as 1,4-hexadiene, dicyclopentadiene, dicyclooctadine, methylenenorbornene, ethylidenenorbornene, tetrahydroindene, etc.). The analogous fluorocarbon, silicone and polysulfide rubbers may also be employed as an EL.
The SP/TE polyblends may be diblends, triblends or even blends of a greater number of constituents, including, polyblend mixtures of one or more suitable TE's. Broadly speaking, the SP/TE polyblends may be comprised of between about 40 and about 95 weight percent of the SP constituent. More often, however, the SP content ranges from about 50-85 weight percent, with polyblends wherein the proportion of SP is in the neighborhood of 80 weight percent being frequently preferred.
There are several known and heretofore disclosed and, to varying extents, employed to cross-link and/or improve the physical properties of SP/TE polyblend systems. These, all quite diversified but ordinarily and usually without significant modification(s) reasonably adaptable to ordinary blend processing (often involving melt conditions) procedures, include:
(1) The use of processing temperature (heat with oxygen, as from air, present) to effect cross-linking the EL in the polyblend system. The amount of cross-linking is affected by the mechanical temperature, (speed generally in revolutions per minute, i.e., "RPM") of the mixing heads and mixing time (i.e., "MT"). In this technique, the following generalities are observable:
Increase of processing temperature.fwdarw.Increased cross-linking PA1 Increase and mixing rate (or RPM).fwdarw.Increased cross-linking PA1 Increase of mixing time.fwdarw.Increased cross-linking
(2) Using peroxide catalysts to cross-link the EL and improving blend properties. Cumene hydroperoxide (i.e., "CHP"); 1,1 bis(t-butyl peroxy)cyclohexane (i.e., "TBPC"); and t-butyl hydroperoxide (i.e., "TBHP") are effective for this. This sort of technology is disclosed in Refs. 3, 4, and 5.
(3) Another known means is the use of beta radiation (as from an electron beam source) to cross-link the EL. This treatment of the prepared resin improves physical properties of the polyblend and appears optimum at about 1/2 megarad (i.e., "MRAD") dosage(s).
Nonetheless, nothing in prior art appears to realistically concern itself with an improved and highly effective means and composing technique for greatly enhancing the important physical properties, especially the ESCR characteristics, by cross-linking effects in SP/TE polyblends to get better and more satisfactory products thereby and as a result thereof in the way so indigenously advantageous as in the present contribution to the art.
______________________________________ LISTING OF REFERENCES Ref. No. Identification ______________________________________ (1) U.S. Pat. No. 3,322,734 (R. W. Rees); (2) U.S. Pat. No. 3,404,134 (R. W. Rees); (3) U.S. Pat. No. 3,429,951 (C. W. Childers); (4) U.S. Pat. No. 3,476,829 (J. T. Gruver and C. W. Childers); (5) U.S. Pat. No. 3,499,949 (C. W. Childers and J. T. Gruver); (6) R. R. Durst, R. M. Griffith, A. J. Urbanic and W. J. Vanessen of the Research and Development Division of THE GENERAL TIRE AND RUBBER COMPANY in a paper presented at the 168th National Meeting for the 1974, September 8-13, of the AMERICAN CHEMICAL SOCIETY; (7) H. L. Morris, "Thermoplastic Elastomers" at pp. 103-104 of MODERN PLASTICS ENCYCLOPEDIA (1976-1977); (8) J. A. Radosta, "Improving The Physical Properties of Impact Polystyrene" at pp. 28-30 of PLASTICS ENGINEERING (September, 1977); (9) "Modification of Polystyrene with SOLPRENE (Reg. TM) Plastomers", PHILLIPS CHEMICAL COMPANY Publication, TR-17; (10) "KRATON (Reg. TM) Rubber Products Brochure", SHELL CHEMICAL COMPANY (March, 1977); and (11) "Shell KRATON (Reg. TM) Rubber For Modification of Thermoplastics", Technical Bulletin No. SC:165-177, SHELL CHEMICAL COMPANY (february, 1977); and ______________________________________
Other references of possible interest (with capsulated descriptions of their contained subject matter disclosures) include:
______________________________________ (12) U.S. Pat. No. 2,537,951--Treating styrene copolymers with minor amounts of diallyl maleate, divinyl benzene, etc., to give so-called "popcorn" polymers; (13) U.S. Pat. No. 2,665,270--Involves copolymers of styrene/- divinyl benzene/ethyl vinyl benzene; (14) U.S. Pat. No. 2,668,806--Same as 2,665,270; (15) U.S. Pat. No. 3,781,382--Concerns making of an impact vinyl aromatic . . . by mass polymerizing until 2-15 percent conversion of monomer . . . with at least one monovinyl aromatic compound having dissolved therein at least one rubbery diene . . . etc.; (16) U.S. Pat. No. 3,912,703--methods of increasing and decreasing molecular weights of internally unsaturated polymers . . . via use of olefins of lower molecular weight and a disproportionation catalyst; (17) J5/1066385 from DERWENT's Plasdoc-Vinyl aromatics are continuously polymerized without gelation in the presence of an inhibitor and small amounts of divinyl benzene; (18) CA 53:31h--Concerns preparation of copolymers of divinyl benzene and styrene in a solvent to control gelation; and (19) Ca 53:8691h--Concerns radiation polymerization of styrene/divinylbenzene copolymers to improve product strength of resulting styrene polymer. ______________________________________
Further possible-interest References of other than United States origin include:
______________________________________ (20) Belgian 805,589--Hexahalogenated cyclopentadiene is employed to treat butadiene/styrene polymers for molecular weight increase. (21) Russian 328,105--Relates to use of divinyl benzene as a viscosity regenerator in the copolymerization of styrenes; (22) Russian 328,106--Relates to copolymers of alpha-chloromethyl styrene and diisopropyl- benzene; (23) Russian 472,133--Concerns use of divinyl benzene as a cross-linking agent for styrene copolymers; (24) German 1,092,204--Deals with cross-linked copoly- mers of styrene and divinyl benzene; (25) CA 44:5151f--As to copolymerization of styrene/- divinyl benzene; (26) CA 50:16175e--Touches on a styrene/divinyl benzene polymerization system; (27) CA 46:10667i--Prescribes viscosity-molecular weight conditions for butadiene/styrene divinyl benzene systems; (28) CA 46:7816h--Studies cross-linking in styrene/- divinyl benzene systems with small quantities therein of the difunctional monomer; (29) CA 32:3049.sup.5 --Compares styrene cross-linked with divinyl benzene with other cross-linking agents; (30) CA 39:5153.sup.9 --As to polymer swelling with such compositions as copolymers of styrene and divinyl benzene; and (31) CA 84:136114g--Involving gelation in the anionic polymerization of divinyl benzene and styrene. ______________________________________